A. Field of the Invention
This invention relates to the manufacture of phosphoric acid. More particularly, the invention relates to an improved method of manufacturing volatile phosphorous compounds and their subsequent conversion to phosphoric acids.
B. Prior Art
Essentially all phosphate deposits, commonly referred to as phosphate rock, in the United States are variants of apatite (Ca.sub.3 (PO.sub.4).sub.2), with the predominant deposits being fluorapatite (3Ca.sub.3 (PO.sub.4).sub.2.CaF.sub.2). Fluorapatite is highly insoluble in its natural form and must be chemically treated with acid to convert the phosphorus to a soluble form. The major market for phosphate rock is as a raw material for fertilizer manufacture, although a substantial market exists for "food grade" phosphoric acid. In the fertilizer industry the phosphorous content is normally referred to as the P.sub.2 O.sub.5 equivalent rather than the elemental phosphorous content.
The simplest treatment for phosphate rock to make the phosphorous available as a plant food is to acidulate with sulfuric acid to produce normal superphosphate, as follows: EQU 3Ca.sub.3 (PO.sub.4).sub.2.CaF.sub.2 +7H.sub.2 SO.sub.4 .fwdarw.3Ca(H.sub.2 PO.sub.4).sub.2 +7CaSO.sub.4 +2HF
The disadvantage of this process is one of materials handling in that the P.sub.2 O.sub.5 equivalent in the product usually averages only around 20.00%; that is, the major constituent is calcium sulfate, which has essentially no value as a product.
Another process used in the phosphatic fertilizer industry is the manufacture of "wet process" phosphoric acid, wherein relatively dilute sulfuric acid is used, but in sufficient quantity to liberate phosphoric acid, as follows: EQU 3Ca.sub.3 (PO.sub.4).sub.2.CaF.sub.2 +10H.sub.2 SO.sub.4 +20H.sub.2 O.fwdarw.10CaSO.sub.4.2H.sub.2 O+6H.sub.3 PO.sub.4 +2HF
The "green" phosphoric acid thus produced, named green because of its color, normally has a concentration of around 32.00% P.sub.2 O.sub.5 equivalent, which can be concentrated by evaporation to higher strengths. The disadvantages of this process are the long residence time and stages required for proper size gypsum crystals to form to facilitate filtration, the need for phosphoric acid recycle to facilitate the reaction, and the extensive equipment required under highly corrosive conditions for separating gypsum from the phosphoric acid and for concentrating the relatively dilute phosphoric acid to the concentrations normally used in commerce.
The major phosphatic fertilizer on the market today is triple superphosphate, produced by the reaction of phosphate rock with concentrated wet process phosphoric acid, as follows: EQU 3Ca.sub.3 (PO.sub.4).sub.2.CaF.sub.2 +14H.sub.3 PO.sub.4 .fwdarw.10Ca(H.sub.2 PO.sub.4).sub.2 +2HF
The P.sub.2 O.sub.5 equivalent of the product is normally around 47.00%. The primary disadvantage of the process is that relatively expensive phosphoric acid is required as the reactant and relatively large amounts of calcium and other impurities are still present in the product. In the presence of moisture and silica usually present, the HF reacts further to form products of negligible value.
Phosphoric acid may also be produced by the electric-furnace elemental phosphorus route through the direct reduction of phosphate rock, followed by oxidation of the phosphorous to P.sub.2 O.sub.5 and its subsequent hydrolysis to H.sub.3 PO.sub.4. However, major economic disadvantages exist in this technology as large quantities of electrical energy are consumed and expensive high temperature furnaces are required.
An alternate process (U.S. Pat. No. 3,402,019), which apparently has not been commercialized, has been proposed whereby phosphate rock is contacted with SO.sub.3, calcium fluoride (or other metal fluoride) added, placed in an enclosed container, heated for several hours at 200.degree. to 600.degree. C., with the reaction products then vented and recovered, generally, in accordance with the following reaction: EQU (phosphate rock.X SO.sub.3)+CaF.sub.2 .fwdarw.CaSO.sub.4 +POF.sub.3
From the vapor product, which contains phosphorous oxyfluoride, phosphoric acid and hydrofluoric acid may be recovered by hydrolysis. If silicon containing compounds are present, silicon tetrafluoride, although having limited commercial value, may also be recovered.
The preparation of phosphoryl fluoride and difluorophosphoric acid by treating phosphate sources such as phosphate rock, phosphoric acid, and metal phosphates with fluorosulfonic acid has been described in U.S. Pat. No. 3,428,422 of Feb. 18, 1969. U.S. Pat. No. 3,429,659 relates to the preparation of the same volatile phosphorous compounds from a fluorosulfonate salt, while U.S. Pat. No. 3,592,594 describes the preparation of phosphorous pentafluoride from phosphoryl fluoride by treating with sulfur trioxide and hydrogen fluoride.